Imaging electron filters are used in transmission electron microscopes in order to improve the contrast of the object image or diffraction diagrams by the selection of electrons of a specific energy range. The registration of element distributions and energy loss spectra are also possible with filter systems of this kind.
U.S. Pat. Nos. 4,740,704 and 4,760,261 disclose filter systems which, as dispersive elements, use three or four homogeneous magnetic fields. These filters are of the direct vision kind, that is, the optical axes of the entering and exiting electron paths are coaxial to each other. For this reason, these filters can be well used in electron microscopes wherein the optical axis of the electron paths usually runs vertically from top to bottom for reasons of stability. The input and output edges of the magnetic pole pieces are inclined to the direction of the wavefront of the electron beam for focusing the electrons in the direction of the magnetic deflection fields whereby quadrupole portions are provided which are effective in the direction of the magnetic field.
The dispersion in the energy selective plane of such a spectrometer is approximately 1 to 2 .mu.m per eV at an energy of the electrons of 100 keV. For this reason, energy resolutions of approximately 1 to 2 eV can be adjusted. The dispersion in the homogeneous magnetic fields drops with increasing electron energy. For this reason, filters with significantly larger dimensions are required for adjusting the same energy resolutions for higher electron energies. For use in electron microscopes, this leads to a considerable lengthening of the electron optical column whereby the mechanical stability of this column is reduced. Furthermore, the geometric imaging aberrations increase at the same time since, for filters having larger dimensions, off-axis rays are lead further away from the optical axis than for filters having a smaller dimension.
Magnetic spectrometers having inhomogeneous deflection fields are, for example, disclosed in the article of James S. O'Connell entitled "Simple Broad-Range Magnetic Spectrometer" published in The Review of Scientific Instruments, Vol. 32. No. 12, December 1961, pages 1314 to 1316 where they are described with respect to the field of electron spectroscopy. The spectrometer has a single sector magnet having pole piece faces inclined toward each other. The gradient field resulting therefrom additionally affects, in addition to beam deflection, a focusing perpendicular to the plane of the electron beam axis. In this spectrometer, the optical axes are, however, not coaxial to each other when entering into or exiting from the filter. This leads to unstable total configurations when used in electron microscopes.
The use of spectrometers with inhomogeneous deflection fields in electron microscopes is, for example, suggested in the article of X. Jiye el al entitled "A Study of an Inhomogeneous Gradient Magnetic Field Spectrometer with a Curvolincar Axis", published in Optik, Vol. 71, No. 2 (1985), pages 73 to 79. This work, however, relates essentially to paths in magnetic fields having different inhomogeneity. Specific suggestions for the configuration of such filters cannot be derived therefrom.
The use of two inhomogeneous deflection fields in electron beam apparatus is suggested in an article of the journal Optik, volume 38, (1973) starting at page 502 and in the article of E. Plies entitled "Korrektur der Offnungsfehler elektronenoptischer Systeme mit krummer Achse und durchgehend astigmatismusfreien Gau.beta.schen Bahnen" published in Optik, Vol. 40 (1974), pages 141 to 160. This system, however, only corrects aperture aberrations. Here also, the optical axes of the electron beams are not coaxial to each other before the entry into the first deflection field and after exiting from the second deflection field.